By Jerry Schappacher, Executive Advisor, J microTechnology, Portland, ORTest Engineering is a critical function in the manufacture and test of semiconductor devices. It is important for productivity (read profitability) that test programs and test probe cards are effectively developed and verified. Without proper verification, good die are not identified, generating low yield, or bad die are not screened out, resulting in either bad yield at packaged-part final test or delivering bad parts to the customer. Neither are good situations.

To mitigate these threats, test engineers need to thoroughly verify the probe cards and test programs against the test requirements of the product specification. They must also be attentive to the potential for design or fabrication defects corrupting the success of their efforts. An important cross functional approach is teaming the design verification test and the test engineering task during product development. To do a comprehensive check-out requires that both the physical fixture and test program (software) be integrated with the electrical parameter test equipment. It requires the ability to visually inspect the probe contact areas on the semiconductor, and sometimes the interconnection metal features, at the microscopic level. The most obvious test setup is the existing unit on the production test floor, where the complete setup that will be used normally resides.

Assembly floor test equipment generally consists of a 1 to 4 million dollar electrical tester connected to a 150 to 250 thousand dollar high throughput, environmentally controlled, automated probe station with cassette-to-cassette load and unload. This equipment is meant to be run three shifts a day with lots of wafers being tested, screened, and pushed on to packaging. To mount the probe card to verify a new product on a high throughput automatic probe station and then perform engineering tasks is an underutilization of an expensive resource. It is also disruptive to the assembly production process. What is needed is a basic probe station with an optical inspection capability that can be easily set up with interfaces to connect to the electrical tester and mechanical interfaces to mount the probe card in a stable fashion. By bringing manual probe stations to the production floor, verification of probe cards and test programs will not slow down production test or generate inefficiencies of equipment use. Expensive automated and automatic environmentally controlled probe stations can be used to maximize test capabilities during the assembly process.

When selecting a manual probe card and test program verification test station to complement automated production floor test equipment, choose a station with an 8-in. chuck, a probe card adapter, and an inspection microscope system. For flexibility, the probe holder should be compatible with both standard 62 mil or stiffened 125 mil 4.5-in. wide probe cards. The station should support test wafers up to 8-in. in diameter or die on expanded rings, have adjustments to fixture the probe card height and planarity, and supply a work surface for trouble-shooting test hardware and software.

Optics with a magnification range of 21X to 135X give the visibility needed to align the die to the probe needles on the probe card and for low level inspection, while an 84X to 540X magnification level furnishes a magnification acceptable for semiconductor defect inspection. Fine Z-height adjustability for the chuck with finer than 1° rotational control is desirable, particularly if a locking mechanism is available to make the wafer more stable. By having a probe card holder that does not interfere with the top plate surface, positioners with probes can be added for trouble-shooting or monitoring signals on test pads. With manual equipment, you can perform these types of tests that are valuable to have, but usually aren't practical to perform on automated equipment during production test. For documentation and inspection of physical defects during the initial probe card and test program development, and to easily share the information, it is convenient to have an inspection microscope capable of 21X to 540X with a USB camera for direct to digital image capture and wide screen image viewing.

Since all semiconductor devices are temperature-sensitive, measurements at non-ambient temperatures are important. Having a manual probe station with a thermal chuck in a production environment enables test engineers to evaluate some of the devices to determine how they would function in normal operating conditions. Although most people look at the production floor environment as one for volume manufacturing and automated assembly, bringing a basic test probe for manual test processes to the production floor has many advantages. Test engineers can verify probe cards, test programs, and perform initial wafer lot qualification, or evaluate wafers and devices as manufacturing is in progress, thus identifying possible problems early. Device defects can be separated from test fixture wiring or programming defects during the engineering phase of development. Documentation and communication is improved with clear digital photographic images available immediately. The result is improved test engineering productivity at a fraction of the cost of using an automated probe station and without tying up production equipment and reducing production line throughput.